Page 82 - Troubleshooting Analog Circuits

Other Strange Things That Diodes Can Do to You . . . 69

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Figure 6.3. Even though the diodes in the first stage of this op amp are forward or reverse biassed by
only a millivolt, the impedance of these diodes is much lower than the output impedance of
the first stage or the input impedance of the second stage at high temperatures. Thus, the op
amp’s gain drops disastrously.

fast turn-on of a diode circuit, with low overshoot. you must keep the inductance of
the layout small. It only takes a few inches of wire for the circuit’s inductance to
make even a good fast rectifier look bad, with bad overshoot.
One “diode” that does turn ON and OFF quickly is a diode-connected transistor. A
typical 2N3904 emitter diode can turn ON or OFF in 0.1 nsec with negligible over-
shoot and less than 1 pA of leakage at 1 V, or less than 10 pA at 4 V. (This diode
does, of course, have the base tied to the collector.) However, this diode can only
withstand 5 or 6 V of reverse voltage, and most emitter-base junctions start to break
down at 6 or 8 V. Still, if you can arrange your circuits for just a few volts, these
diode-connected transistors make nice, fast, low-leakage diodes. Their capacitance is
somewhat more than the 1N914’s 1pF.

Other Strange Things That Diodes Can Do to You.. .

If you keep LEDs in the dark, they make an impressive, low-leakage diode because
of the high band-gap voltage of their materials. Such LEDs can exhibit less than 0.1
pA of leakage when forward biassed by 100 mV or reverse biassed by 1 V.
Of course, you don’t have to reverse-bias a diode a lot to get a leakage problem.
One time I was designing a hybrid op amp, and I specified that the diodes be con-
nected in the normal parallel-opposing connection across the input of the second
stage to avoid severe overdrive (Figure 6.3). I thought nothing more of these diodes
until we had the circuit running-the op amp’s voltage gain was falling badly at 125
“C. Why? Because the diodes were 1N914s, and their leakage currents were increas-
ing from 10 nA at room temperature to about 8 FA at the high temperature. And-
remember that the conductance of a diode at zero voltage is approximately (20 to 30
mS/mA) X ILEAKAGE. That means each of the two diodes really measured only 6 kn.
Because the impedance at each input was only 6 ka, the op amp’s gain fell by a